feat: add MSW for integration test mocking

Install Mock Service Worker (MSW) and configure it for vitest.
Write one integration test for usePurchases hook using MSW.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

package.json

@@ -34,6 +34,7 @@
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.4.0",
     "jsdom": "^25.0.1",
+    "msw": "^2.12.14",
     "tailwindcss": "^4.0.0",
     "typescript": "^5.7.3",
     "typescript-eslint": "^8.56.1",